The invention relates to an improved means for releasably locking a display container of the type sometimes referred to as blister packaging. Particularly, the invention is directed toward releasably securing male and female portions to hold the package in a closed position for display and storage.
Plastic packages for exhibiting a wide variety of articles to be sold have long been used. Some have cardboard or paper backing adhered to the back of a plastic envelope formed in the shape of the goods. Others have front and back plastic sections which are detachably joined along a seam for separating the plastic halves and removing the item. The sections may be adhesively bonded or otherwise joined along the seam to maintain closure of the package. Oftentimes, metal or plastic fasteners and clips secure the package in the closed position.
Another type of plastic involves integrally molded front and back envelopes joined along a pivotable hinge portion. With this type package marginal edges of the front and back envelopes meet when the envelopes are pivoted to the closed position. These edges are typically fastened together mechanically or by adhesive bonding.
More sophisticated approaches to securing packages have included blister-type packs with integral snap-over edges or snap-together elements in which a resilient engagement is achieved. A problem with such fastening means is that very close mold tolerances must be maintained or else the molded fastening portions may not properly mate and securely hold. Since this type of plastic packaging is predominantly manufactured by vacuum, or thermo, forming, a great deal of expense is incurred in making molds which will produce the tolerances required. Thermo forming molds, especially the portions for forming the parts of fastening clasps were required to be precisely made for particular thicknesses of plastic film. Usually, plastic packs are made in the range of from about 19 gauge to about 31 gauge, and the particular thickness used depends upon the size, weight and shape of the article to be containerized. A resiliently engageable design may work well for one gauge but might not be usable with a different thickness. Snap-engagements are in large part reliant upon the modulus of elasticity of the material as well as accurate part dimensions. Thus, changes in the formulation of the plastic material may be required to enable a uniform fastener design to be useful over a range of thicknesses.
Another aspect of making vacuum formed plastic packages involves providing positive mold draft so that the formed item can be released from the mold. Consideration of the draft required at the engageable portions must be given when close tolerances for snap-fits are required. This problem is compounded when proper engagement requires a relatively sharp edged member on one half of the package to resiliently engage a complementary member on the other half. In the past, even these extra cost and design efforts required to form resiliently engageable members have not succeeded in achieving very effective fastening means.
It is therefore a primary goal of the invention to provide improved releasable locking means for plastic packaging, which does not require close mold tolerances to achieve secure resilient engagement.
An important goal of the invention is to provide plastic blister-type packaging having integrally formed resilient locking means of the clasp type, which comprises a zero draft female portion and a snap-engaging zero, or negative, draft male portion.
It is an allied goal to take advantage of a discovered phenomenon in vacuum forming plastics which creates a widened collar at the crown of the male portion and a narrowed rim at the opening of the female portion while the molten plastic is drawn to the mold and thereafter cooled. The collar and rim have been found to provide complementary shapes enabling an enhanced resilient engagement.
Yet another object of the invention is to provide male and female portions which resiliently contact at spaced-apart points during snap-engagement and disengagement.
A subservient goal is to form such locking means as integral portions of a hinged plastic package so that the locking means are manually engageable and disengageable to facilitate closing and opening of the package in a simple manner.
The invention can be summarized as providing an improved resilient locking means for thermo formed plastic packages or containers of the clasp type using "a round peg in a square hole". The improved locking means comprises vacuum forming at least one set of opposingly aligned male and female portions in the plastic sections. The male portion is formed with a zero or negative draft sidewall, circular in cross-section, and has a height no greater than the depth of the female portion. The female portion is formed with a zero draft sidewall which is polygonal in cross-section. During vacuum forming, the drawn molten plastic creates an outwardly projecting collar around the crown of the male portion and an inwardly projecting rim around the opening of the female portion. The uniquely formed collar being slightly larger in outside diameter than an opening bounded by the rim. The collar and rim have geometric outlines that permit them to resiliently snap-engage at symmetrically spaced-apart points. The resilient engagement releasably secures the formed plastic sections to each other. The male and female portions have a common central axis when engaged and the sidewall mold width of the female portion need only be roughly formed at about the same width as the mold for the male portion sidewall taken at the crown thereof.